Using highly purified monospecific antiserum to serum proteins involved in the glomerular reaction in nephritis, attempts are being made to identify in the glomerular ultrastructure the site of antigen deposition. The highly purified antibody is being produced with immuno- absorbants. Labeling will be by mercury, uranyl acetate or peroxidase. The main difficulty is satisfactory reaction of the tissue with antibody. Fixation abolishes or diminishes the reaction of antigen with antibody. To avoid fixation, we are now using tissue that has been freeze dried by azeotropic distillation. With immersion in labeled antibody, good penetration of such tissue has been achieved. Thereafter the tissue will be fixed, embedded and sectioned. Alternatively, it may be possible to cut satisfactory thin sections from tissue embedded in bovine serum albumin gel without fixation followed by reaction of the thin section with antibody.